The invention relates to a method for obtaining a composition in the form of a crumb comprising an aromatic polyamide from an aromatic diamine and an aromatic diacid chloride, which aromatic polyamide comprises 5(6)-amino-2-(p-aminophenyl) benzimidazole terephthalamide units and has a relative viscosity ηrel of at least 3 by copolymerizing 5(6) -amino-2-(p-aminophenyl)benzimidazole (DAPBI) and optionally para-phenylenediamine (PPD), and terephthaloyl dichloride (TDC) in a mixture of N-methyl pyrrolidone (NMP) and calcium chloride.
A method of adding calcium chloride to a solvent in a polymerization mixture or after the start of the polymerization process is known from U.S. Pat. No. 4,172,938. This reference discloses in example 34 the polymerization of a PPD/DABPI/TDC mixture. However, according to this reference all of the calcium chloride was added in the NMP solvent prior to adding the monomers. Further a low DABPI content (10 mole %) polymer was made. This process did not lead to the forming of a crumb, but only a paste-like product was obtained. This reference does further not give any hint that all calcium chloride can be added after having added the monomers to the solution, and when part of the calcium chloride is added after the start of the polymerization process, the CaCl2/aromatic diamine molar ratio must become 0.6-1.0 to obtain crumb.
A method for performing such polymerization reaction but obtaining a composition in the form of a crumb or a crumb-like material was disclosed in WO 2005/054337. According to this method the monomer of interest, DAPBI (5(6)-amino-2-(p-aminophenyl)-benzimidazole), is added to the aromatic diamine mixture with the objective to obtain a suitable polymer solution right after polymerization with e.g. PPD and TDC, which can be directly shaped into fibers or films, whereby DAPBI is seen as a suitable co-monomer to keep the aramid polymer in solution. It was found that by selecting a specific molar ratio of PPD, DAPBI, and CaCl2 the formation of powders, paste, dough, and the like could be prevented. Said method therefore pertains to a method for obtaining an aromatic polyamide comprising 5(6)-amino-2-(p-aminophenyl)benzimidazole terephthalamide units as a crumb with a relative viscosity ηrel of at least 4.
This method, wherein the monomers are added to the final solvent system (CaCl2/NMP) is very suitable for obtaining crumbs and crumb-like materials when performed at the scale as indicated in this patent application, i.e. in a small reaction flask of 2 L.
Unfortunately, it appeared that this method failed when the process was scaled-up. Thus under commercial production conditions in a reactor having a capacity of larger than 50 L, such as in a Drais reactor of 160 L, no crumbs were formed and the material was obtained as a big lump sticking around the stirrer, which was not suitable for further processing. It is therefore an object of the invention to provide a method that is suitable for large scale production for obtaining aramid polymer having DAPBI units with a high relative viscosity and a suitable material consistency. The method of course can also be used for small-scale production of aramid polymer having DAPBI units. When used for such small-scale polymerization reactions the method of the invention can be considered an alternative for the method of WO 2005/054337, wherein the solvent is added prior to polymerization.